As is known, many food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of this type of package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing laminated sheet packaging material.
The packaging material has a multilayer structure substantially comprising a base layer for stiffness and strength, which may comprise a layer of fibrous material, e.g. paper, or of mineral-filled polypropylene material; and a number of layers of heat-seal plastic material, e.g. polyethylene film, covering both sides of the base layer.
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of gas- and light-barrier material, e.g. aluminum foil or ethyl vinyl alcohol (EVOH), which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
As is known, packages of this sort are produced on fully automatic packaging machines, on which a continuous tube is formed from the web-fed packaging material; the web of packaging material is sterilized on the packaging machine, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, once sterilization is completed, is removed from the surfaces of the packaging material, e.g. evaporated by heating; and the web of packaging material so sterilized is maintained in a closed, sterile environment, and is folded and sealed longitudinally to form a vertical tube.
The packaging machines comprise a forming unit, wherein the tube is filled continuously downwards with the sterilized or sterile-processed food product, and is sealed and then cut along equally spaced cross sections to form pillow packs, which are then fed to a folding unit to form the finished, e.g. substantially parallelepiped-shaped packages.
More specifically, the pillow packs substantially comprise a parallelepiped-shaped main portion; and a top end portion and a bottom end portion, opposite to each other and projecting laterally on opposite sides of the main portion and defining respective triangular end flaps to be folded onto the main portion.
A longitudinal sealing strip, formed when sealing the packaging material to form the vertical tube, extends along the pillow packs; and the top end portion and bottom end portion of each pillow pack have respective transverse sealing strips perpendicular to the longitudinal sealing strip and defining respective end flaps projecting from the top and bottom of the pack.
The top end portion and the bottom end portion of each pillow pack taper towards the main portion from the respective end flaps.
Folding units are known, substantially comprising a chain conveyor for feeding pillow packs continuously along a predominantly straight horizontal forming path from a supply station to an output station, and a plurality of folding devices which cooperate cyclically with each pillow pack along the forming path to flatten the respective top end portion and bottom end portion of the pillow pack and so fold the respective end flaps onto the top end portion and bottom end portion.
The folding units comprise heating means arranged for heating the pillow packs and melting the plastic material forming the outer plastic layer of the pillow packs at the top end portion and bottom end portion.
In practice, the pillow packs are usually formed and sealed with their longitudinal axis arranged vertically. The newly formed pillow packs are subsequently cut from the tube and let slide along a curved-profile chute so as to be brought from the vertical position to a substantially horizontal position, in which they are received by a feeding unit, arranged immediately downstream of the chute and which drives the pillow packs to the folding unit.
Once the pillow packs are cut from the tube, they move down to the chute and advance along the chute by gravity.
The feeding unit comprises two guides that extend between an inlet zone, where the packs coming from the chute are received, and an outlet zone, where the packs are delivered to the folding unit.
The feeding unit further comprises a conveyor belt and a plurality of carriers projecting from the conveyor belt and arranged to interact with the packs to advance the packs along the guides.
The conveyor belt is wounded around a first wheel and a second wheel, the first wheel and the second wheel having substantially horizontal axes. In this way, the conveyor belt has an upper active branch and a lower return branch, the carriers of the upper active branch passing through a gap defined between the two guides and pushing the packs.
A drawback of the known packaging machines is that the packs coming from the forming unit may be not synchronised with the feeding unit.
In particular, the packs may be “delayed” due to different factors such as: sticking to the sealing device that carries out the transversal sealing or to the cutter that separates the packs form the tube.
It may therefore happen that—when a carrier reaches the inlet zone—the corresponding pack has not arrived yet and so such carrier is not able to carry any pack. In this case, the following carrier interacts with two packs, i.e. its “own” pack and the “delayed” pack that was not delivered to the preceding carrier. This may cause a jam in the feeding unit or in the folding unit.
It may also happen that—when a pack reaches the inlet zone—the corresponding carrier interacts with an intermediate portion of the pack, instead of an end portion of the pack. In this way, the carrier may crush the pack and damage it.
The above-mentioned drawbacks are particularly relevant in case of high speed packaging machines in which the speed of the conveyor belt is high and the distance between adjacent carriers is small, which makes extremely critical a lack of synchronization between the packs coming from the forming unit and the conveyor belt.